Photomultipliers (PMT) have been known in the field for many years, these being special electron tubes whose task is to detect weak light signals down to single photons by generating and amplifying an electrical signal. A photomultiplier is typically composed of a photocathode and a downstream secondary electron multiplier, configured in an evacuated glass bulb. Since the photomultiplier is a vacuum tube, it is relatively large in size and is, therefore, not suited for miniaturization.
Avalanche photodiodes (APD) are also known in the field, these being highly sensitive and fast photodiodes. They exploit the internal photoelectric effect to generate charge carriers and the avalanche breakdown (avalanche effect) for internal amplification. APDs are regarded as the semiconductor equivalent of photomultipliers and are used, inter alia, in the detection of very low optical power levels and have cut-off frequencies up to the gigahertz range.
For photon counting, APDs are operated in Geiger mode, single-photon avalanche diodes (SPADs) generally being referred to in this context. More recent developments include silicon photomultipliers (SiPM), where a plurality of SPADs are combined in analog or digital fashion to form a larger detection area. Reference is made in this regard merely exemplarily to U.S. Patent Application 2010/0214654 A1, which is hereby incorporated by reference herein in its entirety.
An important consideration in the selection of detection systems is generally a highest possible photon detection efficiency (PDE), a lowest possible dark noise level (caused by what is generally referred to as dark current of individual light-sensitive elements), and a highest possible, respectively maximum photon rate, i.e., a high dynamic response of the detector.
Photomultiplier tubes have a costlier design than detectors fabricated using Si technology. Moreover, photomultipliers generally have a lower photon detection efficiency than avalanche photodiodes, for example. In all types of detectors, the dark noise depends to a significant degree on the total detection area, the quality of the photocathode material, as well as on the operating temperature of the detector.